Due to early rains and saturated soils much of Arkansas' cotton crop has an underdeveloped root system. Such plants are especially vulnerable to nematodes, which can cause yield losses from 100 to 250 pounds per acre.
“Once the ground dried out and the plants began trying to grow with less-than-optimal root systems, root parasites like nematodes really began to show up,” says Terry Kirkpatrick, Arkansas Extension nematologist. “I'm seeing field situations growers blame on the weather. While weather is certainly a component, the chief culprit is actually the nematode.”
There are two types of fields Kirkpatrick is seeing. “First, those that were very early, the leaves are off and it's being picked. Second, I'm seeing cotton that's very late — and many of the late fields are late because nematodes are working the roots over.”
Being a parasite, it's to nematodes' advantage to keep a plant vegetative as long as possible. That's because, when the plant is vegetative, more nutrients are translocated to the roots.
“Once the plant goes reproductive, the nematodes aren't nearly as happy,” says Kirkpatrick. “When cotton reproduces, all energy is directed towards the boll. To put that off, nematodes can delay a crop by a week or two. If you're seeing a strange delay in your crop, it's a good year to take some soil samples.”
The ability to hide behind other suspects (like weather), often keeps growers from properly identifying their crop's real nemesis. “There's no mistaking root-knot damage to the root system — galls form that change both the appearance and functionality of the root. Severely infected plants may have damaged taproots and lateral roots or taproot development may be almost non-existent.”
Nematode-caused root problems grow progressively worse through the season as more and more nematodes attack the plant. Water absorption is significantly decreased in infected plants, and translocation of nutrients is inefficient.
Arkansas has an interesting dispersal of nematodes. The root-knot nematode is found in every county where cotton is grown. In areas with ice-cream soils, root-knot nematodes thrive.
“They just seem to do much better and cause more damage in sweet cotton soils.”
The reniform nematode — which can cause as much damage as root-knot — isn't found statewide yet. That may soon change. Over the last 15 years, it's moved steadily north and spread out.
“Right now, we're still seeing reniform mostly in the southeastern quadrant of the state, south of I-40. The shift is on, though. We've been finding more and more nematodes in the extreme northeast, around Leachville.”
Unlike root-knot, reniform nematodes cause no galls on a plant's roots. Kirkpatrick says apart from smaller root systems (including the taproot), and a yield drag, symptoms are hard to identify.
What about nematicides?
Over the last five years, Kirkpatrick says studies show soil fumigation prior to planting resulted in yield increases between 150 to 220 pounds per acre.
“Whether we're working on reniform or root-knot, the response with nematicides is a little inconsistent. Last year, growing conditions were really good, and responses weren't as high.”
One caveat with the studies: the fields chosen for the tests have “terrible nematode problems. That means fields with less nematode pressure might have shown less dramatic yield improvements. One thing's for sure, though: these studies illustrate how much a nematode problem can cost in yield.”